GtkIconTheme doesn't scale icons beyond the size specified in the theme
anymore. This can result in arbitrarily large menu items when a theme
only provides large icons.
Force icons to always be 16x16 to ensure that menu items always have the
same height.
https://bugzilla.gnome.org/show_bug.cgi?id=741259
I have been convinced that it is a bad idea to change the behaviour
at the same time as deprecating it, so go back to respecting the
Gtk/MenuImages xsetting in GtkImageMenuItem when it is set.
The setting as well as the widget are still deprecated, and the
default value of the setting will remain FALSE.
